The frequent contamination of mammalian cell cultures and virus stocks with mycoplasmata poses a serious problem in biomedical research. The contaminants are difficult to detect, frequently produce no gross readily observable effects and complicate the results of biochemical research on properties of cells and viruses. The purpose of this study is to investigate methods for the elimination of mycoplasmata from cell lines and virus strains. The principal method to be used for cell cultures will be treatment with specific antiserum or specific antiserum and complement. Antibiotics and chemotherapeutic agents will also be tested for effectiveness; the mycoplasmacidal antibiotics are of most interest. Since the biological properties of viruses are much different from those of mycoplasmata, various physical and chemical means of elimination will be tested in viral systems in addition to antibiotics and specific antiserum.